Compressors consist of bent axis type compressors and swash plate type compressors depending on how the rotating force of a driving shaft is converted to the reciprocating motion of a piston. The bent axis type compressors are configured such that a central axis of the piston is inclined with respect to the centerline of the driving shaft. This is so that as the driving shaft rotates, the piston coupled to the end of the driving shaft reciprocates. The swash plate type compressors are configured such that the central axis of the piston is coaxially aligned with the centerline of the driving shaft. This is so that as the driving shaft rotates, the pistons coupled to a swash plate are in contact with the swash plate to thereby reciprocate. The advantage of the bent axis type compressor is that the capacity can be easily increased by increasing an inclined angle of the piston, whereas the disadvantage is that the size is large since the piston is located within a cylinder block of the inclined driving shaft. On the contrary, the swash plate type compressor has the advantage of being compact in size.
Generally, as the pressure of operation fluid in a compressor increases, the operation condition between parts, which are moved relative to each other, deteriorates. Representative examples are an axial unbalance of force, a sharp variation in pressure in a trapping region, and a wear of the cylinder block and pistons.
FIG. 1 is a conceptual view explaining the pressure applied to a valve plate and a cylinder block according to the prior art. Referring to FIG. 1, the bent axis type compressor includes a valve plate 10, a cylinder block 20 arranged in a line with respect to the valve plate 10, and a piston 30 located in a cylinder bore 21 of the cylinder block 20. The cylinder block 20 is relatively rotatable to the valve plate 10. The valve plate 10 includes: inlet and outlet ports 12 and 11 arranged on the left and right sides of an extension line between top and bottom dead centers; an inner seal land 15 disposed inside the inlet and outlet ports 12 and 11; and an outer seal land 16 disposed outside the inlet and outlet ports 12 and 11. The valve plate 10 has a trapping region for switching between intake and discharge at the respective top and bottom dead centers. Two types of axial force act on the valve plate 10 and the cylinder block 20. A pressing force (Fp) of one type is generated so as to force the cylinder block 20 towards the valve plate 10 due to the piston, which moves from the bottom deal point to the top dead center. A separative force (Fse) of the other type is generated due to an oil film applied on the inner and outer seal lands 15 and 16 of the valve plate 10 sliding. If the pressing force (Fp) is greater than the separative force (Fse) when the compressor operates, then the valve plate 10 and the cylinder block 20 may be in contact with each other, thereby causing abrasion. As a result, a torque loss is generated on the compressor. Also, if the separative force (Fse) is greater than the pressing force (Fp), then the valve plate 10 and the cylinder block 20 may be separated away, thus causing working fluids to leak. Thus, the operation efficiency of the compressor is degraded. While actively performing many researches on the reduction in torque loss and fluid leakage, it has been realized that there is a strong need for the development of a novel combination of the valve plate and the cylinder block.
When the piston 30 is positioned at the trapping region while moving from the outlet port 11 to the inlet port 12, if the piston 30 proceeds with its compression, then the internal pressure of the cylinder bore 21 increases rapidly. Further, when the piston 30 is positioned at the trapping region while moving from the inlet port 12 to the outlet port 11, if the piston 30 proceeds with its expansion, then the internal pressure of the cylinder bore 21 decreases rapidly. That is, a rapid pressure variation is generated before and after the trapping region. To prevent such a pressure variation, as shown in FIG. 1, the outlet port 11 is provided with a notch 13 near the top dead center and the inlet port 12 is provided with a notch 14 near the bottom dead center. However, in case of a micro compressor, there are problems since it is not easy to machine the notches 13 and 14 in the valve plate 10, and the machining costs greatly increase.
FIG. 2 is a cross-sectional view showing a piston according to the prior art. Referring to FIG. 2, the compressor includes a piston 50 located within the cylinder block 40 and a swash plate 60. The piston 50 has a body 51 and a shoe 52. The body 51 and the shoe 52 are coupled with a spherical joint. The body 51 is an elongate cylinder that reciprocates in a cylinder bore 41 of a cylinder block 40. The shoe 52 is smoothly rotatable relative to the body 51. As the cylinder block 40 rotates, it moves along the swash plate 60. To increase the amount of intake and discharge of the compressor, the reciprocating distance (i.e., stroke) of the piston 50 should be increased. The stroke of the piston 50 can be increased by increasing a tilt angle of the swash plate 60. However, in such a case, an angle between the shoe 52 and the body 51 also increases. Thus, the piston 50 is in contact with the cylinder bore 41 at points a and b, which are shown in FIG. 2, thereby causing a lateral force. As a result, the cylinder block 40 and the piston 50 become worn out. That is, it is difficult to miniaturize the compressor while maintaining its performance and capacity.